Compressor control system



March 2,1943. B. E. @HAGAN HAL, 2,312,728

` vCOMPRESSOR CONTROL SYSTEM Filed July 12, 1941 "2 Sheets-Sheet l R5alzand THE/I2 ATTORNEY Nw. .WN WN 4..

im u March 2, 1943.

B. E. OHAGAN ET AL COMPRESSOR CONTROL SYSTEM Filed .July 12,v 1941 2Sheets-Sheet 2 INVENToRs and Patented Mar. 2, 1943 COMPRESSOR CONTROLSYSTEM Application July 12, 1941, Serial No. 402,178

7 Claims.

Our invention relates to improved means for controlling a plurality of-air compressors.

In some situations, as for example in interlocking plants wherepneumatic switches are employed, several air compressors are provided tosupply the compressed air to operate the switches. These compressors aredriven by electric motors which are supplied with current over the sametransmission line while they supply compressed air to the same orconnected'reservoirs. These compressors are controlled by pressureswitches so that they are placed in operation when the pressure in thereservoir drops to a predetermined value, and are continued in operationuntil the pressure in the reservoirs is increased to a higherpredetermined value.

It is desirable in situations of this kind that on a reduction in thepressure in the reservoir that all of the air compressors driven by mo-ytors supplied with energy through the same transformer do not attempt tostart up simultaneously as this might place such a heavy load on thetransformer that the voltage of the current supplied through thetransformer would drop so much that the motors would be unable to startthe compressors.

It is desirable also that the compressors be unloaded when they arestarted, that is, that they be not required to compress air against thepressure in the reservoir during the period in which the compressors arebeing started.

It is an object of our invention to provide improved means forcontrolling a plurality of air compressors so that there is a timeinterval between the start of operation of the compressors.

A further object of the invention is to provide a system of the typedescribed and incorporating means to maintain each of the com-V pressorsunloaded during the starting period of the compressor.

Another object of the invention is to provide a system of the typedescribed and which employs standard commercial apparatus insofar as 45l possible.

Other objects of the invention and'features of novelty will be apparentfrom the following description taken in connection with the accompanyingdrawings. Y

We shall describe two forms of compressor control systems embodying ourinvention, and' shall' then point out the novell features thereof inclaims.

ing the compressorcontrol system provided by our invention applied totwo compressors and Fig. 2 is a diagram showing the compressor controlsystem of our invention applied to two compressor units each having twocompressors.

Similar reference characters refer to similar parts in each of the twoviews.

Referring to Fig. 1 of the drawings, there is shown therein a compressorcontrol system embodying our invention. The equipment shown in Fig. 1includes compressors CI and C2 which are driven by alternating currentmotors MI and M2, respectively. The compressors Cl andv C2 may be of anyconstruction well known in` the art and are of a type which do notincorporate-unloading means. The motors Ml and M2 may be any appropriatetype of alternating current motors.'

The compressors Cl and C2 supply fluid under pressure or compressed airto a reservoir R from which air may be drawn for any desired purpose, asfor example to operate the track switches in an interlocking plant on arailroad.

The compressor Cl is connected to the reservoir R through a supply pipeand a check valve Vi which permits air to be supplied from thecompressor to the reservoir and prevents flow of air from the reservoirto the compressor. The compressor C2 is connected to the reservoir Rthrough a check valve V2, the supply pipe, and also through the checkvalve Vl so that air may be supplied from the compressorV C2 through thecheck valves V2 and Vl to the reservoir R., but is prevented fromflowing from the reservoir to the compressor. The check valve V2 alsopreventsair compressed by the compressor CI reaching the compressor C2.

An unloading volume WI is connected to thesupply pipe intermediate thecompressor CI and the check valve VI, while a similar unloadingvolume-W2 is connected intermediate the compressor C2 and the valve V2.

Pressure operated switches P art and their design is not a part of ourinven-` tion. These switches include contacts which are governed inaccordance with thepressure in the reservoir. These contacts are closedwhen the air in the reservoir `is at atmospheric pressure and. remainclosed until the pressure in the reservoir is increased to apredetermined value when they l Vbecome opened. These contacts whenvopen re- In the drawings, Fig. l is a diagram show- -main open until thepressure in the reservoir;

Iv and P2 are prof vided and are connected with the reservoir R. Theseswitches lmay be of.

drops to a value somewhat less than that which caused them to open. Thepressure operated switches also include means by which the pressures atwhich their contacts close and open may be adjustably varied as desired,and it is contemplated that the switches PI and P2 in this system willbe adjusted to operate at dilerent pressures. The switch PI may beadjusted so lthat its contacts open when the reservoir pressure is '70pounds and to close when it is 55 pounds to the square inch. The switchP2 may be adjusted so that its contacts open when the reservoir pressureis 60 pounds and to close when the pressure falls to 45 pounds.

The pressure switch PI has incorpora-ted therein an unloading valvedevice B which controls connection from the supply pipe andi therebyfrom the compressors CI and C2 to ,the atmosphere. The valve B isconnected to the supply pipe intermediate the compressor CI and thecheck valve VI, and intermediate the check valves V2 and VI so that whenthe Valve B is open the compressors C I and C2 and the unloading volumesWI and W2 are vented to the atmosphere, while the check valve VIprevents escape of air from the reservoir R through the valveA B.

The valve B is controlled bythe pressure switch PI so as to be open whenthe contacts ofthe pressure switch are open, and to be closed when theswitch contacts are closed.

A safety valve D is provided. and operatesY in the usual manner torelease uid from the reser- Voir R if the Apressure therein exceeds apredetermined pressure.

The: current for operating the motors MI and M2 is supplied through atransformer T from a suitable source of alternating current theterminals of which are designated BX and CX. This energy may be suppliedovera transmission line, not shown.

The supply of energy from the transformer T to the motors MI and M2 is'controlled by magnetic switches indicated generally by the referencecharacters SI and S2, while a thermal time delay switch Q is employed toprovide a time interval between the supply of energy to the motors MIand M2. In addition, the magnetic switches SI yand S2 each incorporate athermal overload device to protect the motor controlled by the switch inthe event of an overload'.

The switches SI and S2 are providedwith manually operable control leversdesignated LI and L2, respectively. Each of these levers has anintermediate or oil position designated- O, in which it interrupts thecircuit-of the winding of the associated switch to thereby preventoperation of the motor governed by that switch. Each control lever alsohas an automatic position A in which the circuit of the winding of theassociated switch is controlled by the pressure switches in the mannerhereinafter explained in detail. In addition, each of the control levershas a hand position H in which a circuit independent of the pressureswitches ris established to energize the windingv of the associatedswitch and thereby cause operation of the associated compressorregardless of the pressure in the reservoir R.

The equipment is shown in Fig. 1 of the drawings in the condition whichit assumes when, the pressure in the reservoir R: is above 55- pounds tothe square inch so. that contact. IIL of pressure switch Pl and contact.Il of pressure switch P2 are open.Y The equipment is also shown with thevswitch SI interrupts connection from supply wire 2.9 leading from theother terminal of the transformer secondary winding to wire 2| leadingto the motor M'I.

Similarly, contact 24 of switch S2 interrupts connection from supplywire I2 to wire 25 leading to motor M2 and contact 26 interruptsconnection from supply wire 2l) to wire 21 leading to motor M2.

Accordingly, the motors MI and M2 are not operating, while energy is notsupplied to the heating element 30 of thermal relay Q even thoughcontact 3f of switch S2 is closed, since contact I0 of pressure switchP-I is open.

In addition, at this time the unloading valve B is open so that theunloading volumes WI and W2 and the pipes connected therewith are opento the atmosphere.

When` the pressure in the reservoir R drops below 55 pounds, the contactI Il of pressure switch PI closes, while the unloading valve B alsocloses. When contact I U of switch PI is closed, energy is supplied fromtransformer T to the winding of switch SI over the circuit which istraced from one terminal of the transformer secondary winding overlsupply wire I2, contact lil of switch PI, wire I control lever LI, and'contact 33 of the thermal overload device incorporated in switch SI toone terminal of the winding of switch SI, while the other terminal ofthis winding is connected to supply wire 2U `and thus to the otherterminal of the secondaryv winding of transformer T.

On energization of the winding` of switch SI its contacts I5 and I6 pickup and establish connection from the supply wires I2 and 20 to wires I8and 2l leading to motor MI so that this motor operates and drivescompressor Cl. The compressor CI will start readily as the unloadingvolume WI is at atmospheric pressure so there is no air pressure tooppose operation of the compressor. As the compressor is operated, thepressure of the air in the volume WI, the supply pipe, and the connectedpipes builds up, and when it exceeds the pressure in the reservoir R,air llows through the check valve VI to the reservoii. The check valveV2 prevents air supplied from compressor CI at this time from reachingvolume W2 and compressor C2 so that the compressor C2 is maintainedunloaded. The volume WI is proportioned so that the pressure thereindoes not build up appreciably until the compressor has been operatedseveral revolutions and reaches a substantial speed so that the motor MIwill be able to continue to drive the compressor after it is subjectedto load.

On closing of contact I8 of pressure switch PI energy is supplied to theheating element of thermal relay Q over the circuit which is traced fromone terminal of the secondary winding of transformer T through supplywire I2, contact Ill of switch PI, wire I4, control lever LI, wire 36,and contact 3| of switch S2 to one terminal 0f the heating element 3i)of relay Q, while the other terminal of the element 30 is connected tosupply wire 20jand thus to the other terminal of the transformersecondary winding. On the supply of energy to the element 3B is heatsup, and after the expiration of a time interval long enough for thecompressor CI to have substantially reached its full speed the element30 is hot enough tocause contact 38 of device Q to close.

Contact 38 of device Q when closed establishes a pick-up circuit for thewinding of switch S2. through wire I2, contact I of switch PI, wire I4,contact 38 of device Q, Wire 40, control lever L2, and contact 4I of thethermal overload device incorporated in switch S2 to one terminal of thewinding of switch S2, while the other terminal of this Winding isconnected by wire 20 to the transformer T. Energy, therefore, issupplied to the winding of switch S2 and the contacts of this switchpick up and its contact 42 establishes connection from wire I4 to wire4i) to maintain the winding of switch S2 energized after opening ofcontact 38 of device Q. On picking up of the contacts of switch S2 itscontact 3I interrupts the circuit traced above for supplying energy tothe heating element 33 of device Q so that this element cools and itscontact 38 opens.

In addition, on picking up of the contacts of switch S2 its contacts 24and 26 establish connection from supply wires I2 and 23 to wires 25 and21 leading to motor M2 so that the motor M2 operates and drivescompressor C2. As pointed out above, the compressor C2 is uriloaded atthis time so that it will start readily.

When the pressure in the reservoir R increases to 70 pounds, the contactI0 of switch PI opens and interrupts connection from wire I2 to wire I4and thereby cuts 01T the supply of energy to the windings of switches SIand S2 so that the contacts of these switches release and interrupt thecircuits of the compressor motors MI and M2. In addition, on release ofswitch S2 its. contact 42 interrupts the holding circuit for the switchwinding, while its contact 3I closes to permit energy to be supplied tothe heating element 30 of device Q. At this time the unloading valve Bassociated with pressure switch PI also is opened so that the air underpressure in the unloading volumes WI and W2 and in the supply pipe mayescape to the atmosphere.

The control system provided by our invention is arranged so that ifeither motor is obstructed or overloaded, the thermal overload deviceinu corporated in the switch associated with that motor will interruptthe circuit of the switch winding so that the switch contacts willrelease and interrupt the supply of energy to the motor.

If, for example` motor MI is overloaded and draws unusually heavycurrent for an excessive period, the thermal element 35, which isconnected in series with the motor MI will be heated to an abnormaldegree with the result that contact 33 will open and interrupt the'vwinding so that the switch contacts will close and energy will again besupplied to the motor.

If the 'motor now operates freely, it will con- This circuit is tracedfrom transformer T tinue to operate until stopped by operation oi thepressure switch PI or b-y manipulation of the control lever LI. If,however, the overload condition remains, the thermal element will againbe heated and contact 33 will again interrupt the circuit of the switchwinding so that the switch contacts will again release and interrupt theswitch motor circuit. .This cycle will be repeated indefinitely untilthe motor starts and runs, or until operation of the motor is preventedby the pressure switch PI or the control lever LI.

In like manner the motor M2 is protected by the thermal overload devicein the switch S2. This device has a heating element which is connectedin series with the motor circuit and on occurrence of an overload thiselement heats up and causes contact il to open. Contact 4I is includedin both the pick-up and stick circuits for the switch winding so thatwhen this contact is open, the switch winding is deenergized and theswitch contacts release and interrupt the switch motor circuit. Inaddition, on realese of vthe switch contacts contact 3I establishes thecircuit to permit energy to be supplied to the heating element of thedevice Q. If control lever LI is in its automatic position and contactI0 of pressure switch PI is closed, energy will be supplied to thermalelement 3i? and contact 38 of device Q will'close and establish thepick-up circuit for the winding of swit-ch S2 so that the contacts ofthis switch pick up and reestablish the circuit of the motor M2, andalso establish the holding'circuit for the switch winding and interruptthe circuit of the heating element 3l) of device Q..

If the motor M2 starts and operates properly, it will continue to rununtil stopped by pressure switch PI or control lever L2, but if theoverload condition still exists, thermal element 45 will again beoverheated and conta-ct 4I will interrupt the circuit of the Switchwinding, and this cycle will be repeated indefinitely until stopped byswitchPI or control lever LI or L2.

The system provided by this invention `is arranged so that if desiredeither compressor may be started and run at any time regardless of thepressure in the reservoir R.

If it is desired to operate compressor CI, the control lever LI is movedto its position H to thereby establish a circuit independent of thepressure switch PI for energizing the winding of switch SI. This circuitis traced from supply wire I2 through control lever LI, and contact 33of theoverload device to one terminal of the switch winding, while theother terminal of this winding is connected to supply wire 20. Theswitch contacts I5 and i6 therefore close and establish the circuit ofthe motor MI and this motorv -for the motor MI.

When control lever LI is in position H, energy is supplied to theheating element 33 of device Q Yover the circuit which is traced fromsupply wire I2 through control lever LI, wire 36, and contact3l ofswitch S2 to one terminal of element 30, while the other terminal vof`this element isA connected to supply wire 20. The element 30 thereuponheats up so that contact 33 closes. However, closure of contact 38 willnot establish the pick-up circuit for the winding of switch S2 unlesscontact Hl of pressure switch Pl is closed and if at the same timecontrol lever L2 is Ain its automatic position. If .at the time thecontrol lever VLI .is moved to its `position H the control lever L2 ismoved to its off position O, the pickup circuit for switch S2 will notbe established and this switch will remain released so that compresserC2 will not be operated. This arrangement permits a serviceman tooperate only the compressor Cl if` desired so that he can inspect it anddetermine its condition. This arrangement is also such that if thecontrol lever L2 is left in the automatic position when lever LI ismoved to its position H, the switch S2 will not beenergized until afterthe expiration ofthe time delay period determined by thermal switch Q.This prevents starting of the compressors CI vand C2 at the same timeand prevents overloading of the transformer.

The compressor CZ may also be operated whenever desired without regardto the condition of pressure switch P2. When control lever L2 is movedto its hand position H, a circuit is established to energize the windingof switch S2. This circuit is traced from supply wire I2 through controllever L2, and contact 4I of the overload protective means to oneterminal of the winding of switch S2, the other terminal of which isconnected to supply wire 20. The contacts of switch S2, therefore, closeand establish the circuit of the motor M2 so that compressor C2operates. The heating element 45 of the overload protection means isincluded in the motor circuit, while contact M of this means is includedin the circuit of the winding of switch S2 so that the motor M2 isprovided with full overload protection.

When the control lever .L2 is in its position H, no circuit isestablished'to supply energy to the winding of switch Si and this switchremains released.

The pressure switch P2 controls a pick-up circuit for the winding ofswitch S2 so that the compressor C2 will be operated if for any reasonthe compressor is not placed in operation by switch P I, and thepressure in the reservoir R drops to 45 pounds to the square inch.

When the pressure in the reservoir drops t 45 pounds, contact Il ofswitch P2 closes and establishes a pick-up circuit for switch S2,assuming that lever L2 is in its automatic position. This circuit istraced from supply wire I2 through contact l l of switch P2, wire 4G,control lever L2, and contact it of the overload means to one terminalof the winding of switch S2, while the other terminal of this win-dingis connected to supply wire 2t. Accordingly, the contacts of switch S2close and establish the circuit of motor M2 so that the compressor C2operates. and supplies air to the reservoir R.

At this time the motor is provided with full overload protection, whileit will be seen that on closing of the contact il of pressure switch P2the pick-up circuit for switch S2 is complete without waiting foroperation of the thermal time delay device Q. The compressor C2 is,therefore, placed in operation without delay on closing of the contact li of switch P2. This is desirable as the pressure in the reservoir is atan abnormally low value and should be increased without delay.

The pressure switch P2 may be provided with a contact 4% which maycontrol an indication circuit to provide an indication at a remote pointin the event that the pressure in the reservoir falls to the valuee'ective to close the contacts of pressure switch P2.

The system shown in Fig. 1 is intended for use where there are twocompressors. In some situations two compressors are inadequate and oneor more units each having two compressors are provided to giveadditional capacity. These other units are located at other points inthe interlocking installation and the. reservoirs of the various unitsare connected together so that any unit may supply air throughout theentire installation.

In sucha situation it is desirable that the same pressure switch controlall of the compressors so that they will all operate the same amount andthe wear on them will be evenly distributed. Accordingly, the equipmentat one location is arranged so that it controls not only the compressorsat that location but also those at the other locations.

While all of the compressors are controlled by a single pressure switch,it is desired that the compressors at each location do not startsimultaneously as this would overload the transformer through whichenergy is supplied to the motors for the compressors at that location.Accordingly, the control apparatus for the compressors at each locationshould be arranged so that there is a time interval between the time ofstarting of the compressors at that location. The compressors atdifferent locations are supplied through different transformers so thereis no need to providefor a delay in the starting of the compressors atdifferent locations.

Equipment of this type is supplied in complete units which include acase in which are mounted the compressors, motors, pressure switches,magnetic switches, etc., all wired up and ready to operate. Formanufacturing reasons it is desirable that the units for all locations,both controlling and controlled, be substantially the same so that thesame equipment maybe employed for either type of location.

In Fig. 2 of the drawings the control system provided by our inventionis shown applied to two units of the type described. One of these unitsis the controlling unit, and the other is the controlled unit. In thedrawings the controlling unit is shown at the left and the controlledunit at the right. Although only one controlled unit is shown it shouldbe understood that two or more such units may be employed if desired.

The equipment at each of the locations is similar to that at the otherlocations, and is similar also to the equipment shown in Fig. l. At eachlocation there are two compressors which are connected through checkvalves to a reservoir, as explained in connection with Fig. 1, while thereservoirs at the various locations are connected by a pipe 50 fromwhich air may be supplied to the switches or other air consumingdevices. The electric current for the motors for driving the compressorsis supplied over a transmission line, while a transformer is provided ateach location for supplying energy from the high voltage transmissionline to the motors.

'Io identify the equipment at the different locations, the referencecharacters for the equipment at the controlling station, that is theequipment at the left-hand side of the drawings, are followed by thesuiix a, while the reference characters for the equipment at thecontrolled or subsidiary location, that is the equipment at theright-hand end of the drawings, are followed by the surlix b.

The control levers LI and L2 at the various stations serve to conditionthe equipment at'that location for local or remote control or to preventoperation of the equipment altogether. Each lever has a local positionLO in which it conditions the equipment at that location to becontrolled by the pressure switch at that location, and a remoteposition RE in which the equipment at that station is controlled by thepressure switch at another station. Each control lever also has an oliposition O in which it maintains the compressors at that location idle.

In addition, each unit is provided with a connector 5I which may beconnected between the local contacts LO and the remote contacts RE ofthe control levers. This connection is installed in the unit which is toserve as the controlling unit, but is removed from the units which areto serve as subsidiary or controlled units.

The pressure switches PIa and P2a at the controllocation are adjusted toperate at the same pressures as the switches in the system shown inFig. l. Thus the switch PI opens when the reservoir pressure reaches 'I0pounds and remains open until the pressure falls to 55 pounds whereuponit closes and remains closed until the pressure is built up to70 pounds.The switch P2a opens at 60 pounds and closes at Ll pounds. The pressureswitch PIb at the subsidiary location is adjusted to operate at slightlydifferent pressures than the switch Pla at the control location. Thusthe switch PII) opens at 63 pounds or slightly before Pia while itcloses at 57 pounds or slightly before Pia. The pressure switch P2bf maybe adjusted to operate at the same pressures as switch P2a.

If there are subsidiary locations other than that shown, the switchescorresponding to PIb are adjusted to operate at the same pressures asswitch PIb, that is, before PIa operates.

, The equipment is shown in the condition it assumes when the pressurein the reservoirs Ra and Rb-exceeds 55 pounds. At this time, thecontacts of the pressure switches are open, while the unloading valvedevices Ba and Bb are open and connect the unloading voliunes toatmosphere. In addition at this time, the contacts of the switches' Sla,S2a, SIb, and S21) are released and interrupt the circuits of thecompressor motors so that neither of the compressors is operating.

When the pressure in the air in the reservoirs Ra and Rb falls to 57pounds, switch PIb closes and closes the associated unloading valve Bbto thereby cut oi communication from unloading volumes WIb and W2b tothe atmosphere. At this time, contact I0 of switch PIb is closed, but ithas no effect as control levers LI b and L2b are in their remotepositions.

When the pressure of the air in the reservoirs Ra and Rb drops to 55pounds, switch Pla closes and closes the associated unloading valve Ba.At this time, contact Ill of switch PIa closes and establishes thecircuit of the winding of switch SIa, and also establishes the circuitto supply energy to the heating element 30 of time element device Qa. Inaddition, on picking up of contact I0 of switch Pla, energy is suppliedto the windsupply wire I2, contact II) of switch PIa,'control lever Lla,contact 33 of the overload device, and winding of switch Sla. to supplywire 26 leading to the other terminal of the transformer. Accordingly,the contacts I5 and I6 close and 'establish the circuit to supply energyto the motor MIa so that this motor operates and drives compressor Clo.The compressor Cla is unloaded at this time as volume WIa `has beenconnected to the atmosphere so the compressor will start readily.

The circuit for supplying energy to the heating element 30 of device Qais traced from supply wire l2, contact I0 of switch Pa, control leverLIb, heating element 30, and contact 42 of switch S2a to supply wire 2t.Accordingly, the element 39 heats up, and causes contact 38 to close, toestablish a pick-up circuit for the winding of switch S211, while onpicking up of the contacts of this switch contact i2 interrupts thecircuit of heating element 39 and contact 3| establishes the holdingcircuit for the winding of switch S2a. The pick-up and holding circuitsfor switch S2a include the contact 4I of the overload protective deviceso that the motor M2a is fully protected. As a result of picking up ofthe contacts of switch 52a, the circuit of the motor M2a is complete andthis motor drives compressor 02a. The compressor 02a is unloaded when itis started as volume W20. is at atmospheric pressure.

At the time contact Ii) of switch Pa closes, energy is supplied to thewinding of switch Sib over the circuit which is traced from control wireI2 through contact `Ill of switch Pla, jumper 5I, control wire 55,control lever Lib, contact 33 of the overload device in switch SI b, andwinding of switch SIb to supply wire 20' which is common to bothlocations. Accordingly, the winding of switch SIb is energized and thecontacts` I5 and I6 of this switch close and establish the circuit toenergize motor M I b from transformer Tb so that motor MIb drivescompressorlCIb. This compressor is unloaded as volume WIb is atatmospheric pressure, while on operation of the compressor the pressurein this volume builds up promptly as unloading valve Bb is closed sinceswitch PIb is assumed to be adjusted to close at a slightly higherpressure than switch Pla.

On closing of contact I0 of switch Pla, energy is also supplied over thecontrol wires to heating element 30 of device Qb. The circuit forsupplying energy to the element 3!) is traced from supply wire I2 overcontact IEI of switch PIa, jumper 5I, control Wire 55, control leverL21), element 3U, and contact 42 of switch S2b to common supply wire 20.Element 30 thereupon heats up and causes Contact 38 to close andestablish a pick-up circuit for supplying energy over the control wires55 and 20 to the winding of switch S2b, while on picking up of thecontacts of this switch, contact 42 interrupts the circuit of element30, and contact 3l establishes the holding circuit to maintain switchS2b energized by current supplied over the control wires. In addition,on picking up of the contacts of switch S2b; its contacts 24 and 25establish the circuit for supplying energy from the transformer Tb tothe motor M2b, and this motor drives compressor C212. The compressor C2bis unloaded at this time as volume W2b is at atmospheric pressure sothat the compressor starts readily.

From the foregoing, it will be seen that on closing of pressure switchPIa, energy is supplied Yto the switches Sla and Slb so that compressorsCIa and CIb start at once, while energy is also supplied to the heatingelements 30 of- Ytime -delay devices Qa and Qb so that after a period,these devices establish the circuits of the switches 52a and S2b andthereby cause the compresso-rs C211 and C2b to operate.

While the compressors Cia and CI b start simultaneously, this is notobjectionable as the motors for driving them are supplied from differenttransformers and there is no danger of overloading the transformers.Similarly, the compressors CZa and C2b start at about the same time, butthese are also supplied from different transformers. The transformers Taand Tb are both supplied from the same transmission line, but thecapacity of this line is such that it is not aifected materially by theload represented by two compressors starting at the same time.

Each of the motors is provided With complete overload protection as thecontact of the overload device associated with the switch governing thatmotor is included in the circuit of the winding of that switch.Accordingly, if any motor is overloaded, the circuit of the switchcontrolling that motor will be interrupted and the switch will interruptthe supply of energy to the motor.

The overload protection for each motor is independent of that for theother motors and cutting off. of the supply of energy to one motor willnot affect the others. The supply of energy over the control wires 2!!and 55 is entirely independent of the switches at the control locationso that release of either of these switches does not affect the switchesat the controlled or subsidiary location. Accordingly, if any motor isoverloaded, its circuit will be interrupted, but the other motors willcontinue to operate in the normal manner.

When the pressure of the air in the reservoirs Ra, and Rb increases to68 pounds, switch Pib opens and opens unloading valve Bb. Thisvalve hasa relatively small port so that air escapes through it slowly.Accordingly, as compressors Clb or C2b continue to operate most of theair compressed by them will be supplied to reservoir Rb and only alimited amount will escape through the unloading valve Bb to theatmosphere.

1t is desired that valve Bb open while the compressors Clb and C2b arestill operating to insure that the compressors will be unloaded whenthey cease to operate. Y

When the pressure in the reservoirs Ra and Rb increases to 70 pounds,the contact l 0 of pressure switch P l a opens, while unloading valve Baopens. Opening of contact I of switch Pla cuts 01T the supply of energyto the windings of switches Sla, SIb, S2a and SZbso that the contacts ofthese switches release and cut off the supply of energy to the motors sothat they cease to operate.

As a result of opening of unloading valve Ba, the air in the unloadingvolumes Wla and W2a escapes to the atmosphere, While on cessation ofoperation of the compressors Clb and C2b, the air in the volumes Wl band WZb escapes to atmosphere through the unloading valve Bb. Because ofthe restricted size of the port in this valve, some time is required forthese volumes to reduce to atmospheric pressure, but they have ampletime to do so before the compressors operate again.

In this modification ofthe invention, the pressure switches P2a and B2bserve only to control indication circuits to provide an indication ifthe pressure in the reservoirs falls substantially below that to whichthe switch Pla is intended to respond.

If it is desired to change the units so that. the

unit attire right becomes the controlli-ng unit and 775 that at the leftbecomes the controlled or sub-v sidiary unit, the jumper 5l is removedfrom the left unit andl is installed in the right unit. In addition, thepressure switches Pla and Plb are readjusted so that switch Pla opens ata lower pressure than switch Plb, and closes at a higher pressure thanthat switch. A restriction or choke should also be installed in theunloading valve Ba, while the control levers Lib. and L2b should` bemoved to their local positions LO, and the control levers Lla and L2ashould be moved to their remote positions RE.

With the apparatus arranged as described above on a drop in the pressurein the reservoirs Ra and Rb, switch Pla will close and close unloadingvalve Ba. Closure of contact lo of pressure switch Pia does not have anyeffect as. jumper 5| has been removed at this location and as controllevers Lla and Llb are in their remote positions.

When the pressure in the reservoirs Ra and Rb drops further, pressureswitch Plb closes and closes unloading valve Bb. Closing of contact l0of switch Plb establishes a circuit to supply energy to the switch SIband to heating element 3i) of device Qb as'the control levers Llb andL2b are in their local positions LO, while it also establishes a circuitto supply energy over the control wires 23 and 55 tothe switch Sla andto element 33 of device Qa. As a result, the compressors Cb and Ca startat once, While the compressors C2b and 02a start after a time interval.When the pressure builds up so that switch Plb opens, operation of thecompressors is discontinued as explained above.

It will be seen, therefore, that the modication shown in Fig. 2 isarranged so that either unit4 may serve as either the controlling unitor as a controlled or subsidiary unit. Furthermore, little changein theadjustment of the equipment is required, while no change is required inthe equipment itself to change a unit from a controllingto a subsidiaryunit. This enables all of the units to be constructed alike and thenemployedwherever desired.

Although we have herein shown and described only two forms of compressorcontrol systems embodying our invention, it is understood that variouschanges and modifications may be made therein within the scope of theappended claims without departing from the spirit and scope of ourinvention.

Having thus described our invention, what We claim is:

1. In combination, a reservoir, a first and a second compressor forsupplying uid to said reservoir, a first motor for driving the firstcompressor and a second motor for driving the second compressor, a rstmagnetic switch effective when energized to establish av circuit tosupply energy to the rst motor. a second magnetic switch effective whenenergized to establish a circuit to supply energy to the second 'motor`a time delay device eifective when supplied with energy to establishafter a.Y predeterminedv time interval a circuit over which energy maybe supplied to the second magnetic switch, a pressure switch responsiveto the pressure of the iiuid in said reservoir, said switch beingeffective on a predetermined decrease in said Vpressure to s up plyenergy to the winding ofv said first magneticV switch, to said timedelaydev-ice and to establish the circuit controlled by said time delaydevice for supplying energy to the winding of said second magneticswitch. acontrol member governing the circuit of the winding of saidfirst magnetic switch, said control member having a first position inwhich it permits the circuit governed by the pressure switch forsupplying energy to the first magnetic switch to be established, asecond` position in which it interrupts the circuit of said winding, athird position in which it establishes a circuit independentrofsadpressure switch for supplying energy to said winding, a second controlmember governing the circuit of' the winding of the second magneticswitch, said second control member having a first position in which itpermits the circuits governed by the time delay device, the secondmagnetic switch and the pressure switch forsupplying energy to saidwinding to be established, a second position in which it interrupts thecircuit of said winding, and a third position in which it establishes acircuit independent of said pressure switch and of said time delaydevice forsupplying energy to said winding.

2. In combination, a reservoir, a rst compressor for supplying fluid toa supply pipe from which fluid may be supplied through a first checkvalve to the reservoir, .a second compressor for supplying fluid througha second check valve to said supply pipe, a first motor for driving therst compressor, a second motor for driving the second compressor, a rstmagnetic switch effective when energized to establish a circuit tosupply energy to the first motor, a second magnetic switch effectivewhen energized to establish a circuit to supply energy to the secondmotor, a time delay device effective when supplied with energy toestablish after a predetermined time interval a circuit over whichenergy may be supplied to the second magnetic switch, a pressure switchresponsive to the pressure of the fluid in said reservoir, said switchhaving a contact which is closed on a predetermined decrease in thepressure in said reservoir and is elective when closed to establish acircuit to supply energy to the winding of the first magnetic switch, tosaid time delay device and to the circuit controlled by said time delaydevice for supplying energy to the winding of said second magneticswitch, said pressure switchcontact being opened on a predeterminedincrease in the pressure in said reservoir, and an unloadingvalve devicecontrolling communication between saidsupply pipe and the atmosphere,said valve device being governed by the pressure switch so as to b eopen when the switch contactisopen andto be closed when the switchcontact is closed.

3. In combination, a reservoir, a first and a second compressor forsupplying fluid to said reservoir, a first motor for driving the firstcompressor, a second motorfor driving the second compressor, a rstmagnetic switch effective when energized to establish a circuit tosupply energy to the rst motor, a second magnetic switch effective whenenergized to establish a circuit to supply energy to the Second motor, atime delay device effective when supplied with energy to establish aftera predetermined time interval a circuit over which energy may besupplied to the second magnetic switch, a first pressure Switchresponsive to the pressure of the fluid in said reservoir, said switchbeing effective on a reduction in the pressure in the reservoir to onepredetermined pressure to establish a circuit to supply energy to thewinding of said first magnetic switch, to said time delay device, and tothe circuit controlled by said time delay device for supplying energy tothe winding of said second magnetic switch, and a second pressure switcheffective on a reduction in the pressure in said reservoir to a valueless than said one predetermined pressure to establish a circuitindependent of said rst pressure switch and of said time delay devicefor supplying energy to the winding of the second magnetic switch.

4. In combination, a reservoir, a first compressor for supplying fluidto a supply pipe from which uid may be supplied through a rst checkvalve to the reservoir, a second compressor for supplying fluid througha second check valve to said supply pipe, a rst motor for driving thefirst compressor, a second motor for driving the second compressor, arst magnetic switch ef fective when energized to establish a circuit tosupply energy to the first motor, a second magnetic switch effectivewhen energized to establish a circuit to supply energy to the secondmotor, a time delay device effective when supplied with energy toestablish after a predetermined time interval a circuit over whichenergy may be supplied to the second magnetic switch, a first pressureswitch responsive' to the pressure `of the fluid in said reservoir, saidswitch having a contact which is closed on a decrease in the pressure inthe reservoir to one predetermined pressure and is effective when closedto establish a circuit to supply energy to the winding of the firstmagnetic switch, to said time delay device, and to the circuitcontrolled by said time delay device for supplying energy to the windingof said second magnetic switch, said pressure switch contact beingopened on a predetermined increase in the pressure in said reservoir, anunloading valve device controlling communication betweenl said supplypipe and the atmosphere, said valve device being governed by thepressure switch so as to be open when the switch contact is open and tobe closed when the switch contact is closed, and a second pressureswitch effective on a reduction in the pressure in the reservoir to acompressor units each having avrst anda secondl compressor supplyingfluid to said reservoir, each unit having a first motor for driving thefirst compressor and a second-motor for driving thev second compressor,each unit having associated therewith a transformer from which energymay be supplied to the motors of such unit,` each unit having a rstmagnetic switch effective when energized to establish a circuit tosupply energy from the transformer for such unit to the rst motor ofsuch unit, each unit having a second magnetic switch effective whenenergized to establish a circuit .to supply energy from the transformerfor such unit to the second motor of such unit, each unit having a timedelay device effective when supplied with energy to establish after apredetermined time interval a circuit over which energy may be suppliedto the winding of the second switch of such unit, and a pressure switchresponsive to the pressure of the fiuid in said reservoir, said switchbeing effective on a predetermined decrease in the pressure in saidreservoir to supply energy to the winding of the first magnetic switchof each unit, to the time delay device of each unit, and to the circuitscontrolled by the time delay devices for supplying energy to thewindings of the second magnetic switches of each unit- 6. Incombination, a reservoir, a first and a second compressor unit eachhaving a first compressor supplying fluid to a supply pipe from whichfluid may ow through a check valve to the reservoir and a secondcompressor supplying fluid through a check valve to the supply pipe forsuchV unit, each unit having a first motor for driving the firstcompressor and a second motor for driving the second compressor, eachunit having associated therewith a transformer from which energy may besupplied to the motors of such unit, each unit having a rst magneticswitch effective when energized to establish a circuit to supply energyfrom the transformer for such unit to the first motor of such unit, eachunit. having a second magnetic switch effective when energized toestablish a circuit to supply energy from the transformer for suchunitto the second4 motor of such unit, each unit having a time delay deviceeffective when supplied with energy to establish after a predeterminedtime interval a circuit over which energy may be supplied to the-windingof the second switch of such unit, each unit having a pressure switchresponsive to the pressure of the fluid `in said reservoir, the pressureswitch associated with the rst unit being effective on a decrease in thepressure in said reservoir to onepredetermined value to sup-- ply energyto the winding of the rst magnetic switch of each unit, to the timedelay device of each unit, and to the circuits controlled by the timedelay devices for supplying energy to the windings of the secondmagnetic switches of each unit, the pressure switch Iassociated withsaid first unit being effective on. an increase in the pressure in saidreservoir to a second predetermined value to interrupt the supply ofenergy to the windings of the switches and to the time delay devicesofreach unit, each unit having an unloading valve governed by thepressure switch for such unit and controlling communication from thesupply pipe of such unit to the atmosphere, the valve for the rst unitbeing open when the` switch interrupts the circuits controlled therebyand being closed when the switch establishes the circuits controlledthereby, the valve and pressure switch for the second unit beingarranged and adjusted so that on a decrease in the pressure in thereservoir the valve for the second unit closes before the valve for thefirst unit closes and so that on an increase in the pressure in thereservoir the valve for the second unit opens before the valve for thefirst unit opens.

7. In combination, a reservoir, a firstv and a second compressor uniteach having a first and a second compressor supplying fluid to saidreservoir, each unit having a first motorl for driving the firstcompressor and a second motor for driving the second compressor, eachunit having a transformer associated therewith,each unit having a rst.magnetic switch effective when energized to establish a circuit tosupply energy from the associated transformer to the first motor and aseciondv magnetic switch effective when energized to establish a circuitto, supply energy from the transformer to the second motor, each unithaving a first control lever governing the circuit of the winding ofthe. associated first magnetic switch and a second. control levergoverning the circuit of the winding of the associated second magneticswitch, each lever having a local position in which it connects oneterminal of the winding of the controlled switch to-a local contact, aremote position in which it connects the terminal of said winding to aremote contact, and an off position in which it interrupts the'circuitof the switch winding, each unit having a pressure switch responsive tothe pressure of the fluid in saidreservoir, each pressure switch havinga contact whichv is opened when the pressure in the reservoir increasestoY a predetermined value and is closed` when the pressure in thereservoir decreases toapredetermined value, thel pressureswitchcontactof each unitwhen closed establishing connection from one terminalof a source to the local. contact of that unit, each unit having a timedelay device effective when supplied withk energy to establish after apredetermined time interval a circuit over which energy'may be suppliedto the winding of the associated second switch and a control wireextending between the remote contacts of said units, eachA unit. beingadapted to have. a removable connection installed between its local andremote contacts,l the first unit: having such a. contact installed,whereby when. the .control levers of the first unit. are in their local'positions and the control levers of the second unit are in their remotepositions land the contact of the pressure. switch for said first.uniti's'closed energy "is supplied to the winding of the first switch ofthe firstv unit, to the time delay device of such unit. andto thecircuit governed bysaid time delay device .for supplying energy totheWinding of the second switch of, saidi first unit, and whereby energy issuppliedover said". control wires'- tov the winding of thel first switchof the second unit,

to the time delay device ofthe second unit and to the circuit governedby said time delay' device for supplying energy to the-winding of thesecond switch of saidl secondi unit.

` BERNARD E. OHAGAN.

ALLAN 'If'. JOHNSTON.

